The present invention relates to a method of operating an inductive distance sensor in which a first coil is acted on by a defined sinusoidal voltage U.sub.e1 and a second coil, which is connected electrically in series with the first coil and is spaced from it, is acted on by a sinusoidal voltage of the same value but opposite phase U.sub.e2. An element of magnetic material, which may be shaped as a tang or tongue, changes its relative distance from the coils and thus their inductances L.sub.1 and L.sub.2, the tongue being positioned between the coils under the influence of a measurement variable. A voltage U.sub.s with respect to zero potential is tapped off between the coils to serve as a useful signal.
A distance sensor of this type is known from U.S. Pat. No. 4,269,070 wherein the tongue is mounted separately from the coils to a beam, such as a bogie beam, to move relative to the coils in response to a bending of the beam, the bending constituting the measurement variable. It is used, inter alia, to determine the state of load of airplanes in which the load is determined by the shear flexure of the wheel axles or other supporting parts of the landing gear. For the evaluation of the signal, use is made either directly of the voltage U.sub.s or else of the portion U.sub.se thereof which is of the same phase as U.sub.e1. In this connection, the result of the measurement can only be free of error if, on the one hand, the sinusoidal voltages U.sub.e1 and U.sub.e2 are exactly the same and precisely in phase opposition and, on the other hand, it can be assumed that the ohmic resistances in the two coil branches are exactly the same and do not change with temperature. Said conditions are not present in practice so that the signals obtained with such distance sensors are always subject to a greater or lessor error.
For uses in which the measurement error must not be greater than .+-.1%, it is, therefore, necessary to analyze the potential sources of error in order to seek possibilities of avoiding them or of compensating for them.
It is an object of the present invention to avoid errors connected to the use of U.sub.s or U.sub.se as measurement signal and to compensate for errors which are due to the ohmic resistances and their temperature-dependent change and are unavoidable despite careful manufacture and adjustment of the distance sensors. Furthermore, compensation is also to be effected of errors which result from the fact that U.sub.e1 is not precisely equal to U.sub.e2, that U.sub.e1 and U.sub.e2 are not precisely 180.degree. out of phase, and/or that he sinusoidal voltages U.sub.e1 and U.sub.e2 differ from the pre-established value.